1. Field of the Invention
The present invention generally relates to apparatus for making a printing plate and a printing plate thereof and, more particularly, is directed to an apparatus for making a printing plate and a printing plate thereof suitable for printing, such as a gravure printing and so on.
2. Description of the Prior Art
A wide variety of printing methods are proposed for printing and a variety of printing plates, such as relief printing, offset printing, intaglio printing, silk screen printing or the like are employed. Particularly, when pictures such as photographs and so on are printed on a large number of copies at high speed, the intaglio printing is used.
The assignee of the present application has previously proposed an inexpensive apparatus for making a printing plate and a printing plate thereof. The previously-proposed apparatus and the printing plate thereof can provide a printing of high quality and are suitable for the printing of medium quantity of papers so that they can be made for personal use or for office use (see U.S. Pat. No. 5,126,531.)
FIG. 1 shows a conceptual diagram of an optical system of such previously-proposed apparatus for making a printing plate. According to this conventional apparatus, a small energy emitting type semiconductor laser 1 of about 1 Watt is employed to form holes 3 on a printing plate 2.
As shown in FIG. 1, an input image signal 4 from an image scanner or the like is supplied to the semiconductor laser 1, in which it is directly modulated by turning on and off the semiconductor laser 1 by the image input signal which results from pulse code modulating (PCM) a drive current. For this reason, a laser beam emitted from the semiconductor laser 1 is turned on and off in synchronism with the image signal.
Referring to FIG. 1, the laser beam from the semiconductor laser 1 is collimated by a collimator lens 5 and introduced through an objective lens 6 into the printing plate 2 so that the laser beam is focused on the surface position of the printing plate 2. The semiconductor laser 1, the collimator lens 5 and the objective lens 6 constitute a laser block 14, and this laser block 14 is located so as to focus the laser beam on a plate cylinder 8 at its predetermined position on the leftmost side. The plate cylinder 8 is rotated in the direction shown by an arrow B in FIG. 1 by a plate cylinder rotating motor (not shown) coupled to a plate cylinder shaft 9 so that, when the plate cylinder 8 is rotated once, the holes 3 of one track along the circumference of the plate cylinder 8 are scattered by the laser beams to thereby form the holes 3 of predetermined one track amount. Then, if the laser block 14 is moved in the axial direction of the plate cylinder 8 by the amount of one pixel to allow the laser beam to scan the surface of the printing plate 2, the predetermined holes 3 are formed over two tracks. Therefore, if such scanning of laser beam is sequentially carried out on the whole surface of the plate cylinder 8, then holes 3 corresponding to the light and shade (i.e., image information) of the input image signal 4 are formed on a synthetic resin material coated on the surface of the printing plate 2.
When a printed product is produced by using the printing plate 2 and a screen while overlapping inks, such as a cyan. (C), magenta (M), yellow (Y). black (B) or the like, a dark and light stripe pattern called moire appears in various shapes. In order to prevent moire from occurring, angles at which the printing plates 2 are attached to respective color printing plates are adjusted variously so as to make the moire inconspicuous. For example, the angles of the printing plates 2 of deep colors such as cyan, magenta, black or the like are set to be 30 degrees in which the moire becomes inconspicuous relatively such as when the angle of the black printing plate 2 is 45 degrees, the angle of the magenta printing plate 2 is 75 degrees, the angle of the yellow printing plate 2 is 90 degrees and the angle of the cyan printing plate 2 is 105 degrees. If the attaching angle is 15 degrees, the moire tends to become conspicuous as compared with the attaching angle of 30 degrees. However, if the yellow printing plate of low color concentration is disposed between the cyan and magenta printing plates (in this case, the yellow printing plates are disposed at the angle of 15 degrees relative to the cyan and mageta printing plates), the moire can be made inconspicuous.
The pattern of the hole 3 formed on the printing plate 2 by the conventional apparatus for making a printing plate is hyperelliptic as shown by reference numeral 10 in FIG. 2. By way of example, the major axis length of this hyperelliptic pattern 10 is 150 .mu.m and the minor axis length thereof is about 5 .mu.m. In the present invention, by making effective use of the fact that the pattern of hole 3 is hyperelliptic, it is possible to obtain the printing plate in which the shape, pitch or angle of the pattern 10 of the hole 3 formed on the printing plate 2 are variously changed only by adjusting the optical system of the apparatus for making a printing plate.